This invention applies to the field of optical fiber manufacturing, and in particular to manufacturing thermoplastic optical fibers having a low refractive index cladding over a thermoplastic core of a higher index of refraction. Such fibers as large as 3 mm in diameter are finding use in display and sign illumination, as they are capable of transmitting high light levels. The ability of these relatively large diameter fibers to transmit enough light to produce useful levels of visible illumination without any infrared or ultraviolet energy makes them ideal for display lighting in such applications as museums and retail stores. Presently-known plastic optical fibers are manufactured by one of two different and relatively expensive processes.
In one such manufacturing method individual fibers are "drawn" from miniature dies from which a polymethyl methacrylate (acrylic) core and a fluorinated polymer cladding are simultaneously drawn at a fairly slow rate. Plastic fibers are thus manufactured in a continuous process very much like glass fibers have been made for many years. The transmissivity of the fibers is partially dependent on flaws or density variations in the core material, and by surface flaws at interface at the surface of "total" internal reflection between the core and the cladding. Since the fibers are partly extruded from the die and partly drawn from it, variations in both density and diameter are produced by the slight variations in speed of drawing. Also, the very small-diameter holes in the dies through which the fibers are drawn must be drilled with either diamonds or lasers, and it is difficult to produce smooth, highly-polished holes. Thus any slight flaw or imperfection in the drawing die aperture is replicated in the drawn fiber and may cause a surface imperfection that functions as a light leak out of the core, whereby the internal reflection becomes less than total. This method for drawing optical fibers is a slow and precise process, and the resulting cost of clad acrylic plastic fibers of 2 mm to 3 mm diameter needed for display illumination is presently in excess of $60 per pound of finished fiber material.
A second presently-used process for manufacturing plastic optical fibers produces what is know as "solid-core" fibers, in which a core of very soft and flexible polymer is extruded into a sheath of a fluorocarbon plastic which in part traps an air film about the core to provide the low index of refraction cladding. Solid core fiber extrusion has all the manufacturing errors of clad acrylic fibers for the core, but additionally has a similar set of manufacturing errors for the pre-extruded sheath, requiring highly-polished small diameter surfaces for both the core and the sheath dies. As a result, solid core fibers are normally less transmissive than clad acrylic fibers of the same diameter and length. Further, the solid core process produces light guides of limited length, as it is not a continuous process. Thus, manufacturing solid core fibers is an even more expensive process than acrylic fiber drawing. The cost of producing fibers of 3 mm diameter for display lighting is typically $250 per pound of 3 mm diameter core material.
The manufacturing flaws and high costs of plastic optical fiber extrusion have been significantly reduced in other optical plastic extrusion processes, such as those used for manufacturing prismatic light control lens sheets for the fluorescent fixture industry. Light control lenses having linear prisms are extruded in sheet or profile forms, much like optical fibers are extruded from acrylic polymers such as those used for optical fibers, but from a wide extrusion die instead of a small hole. Such profile dies are also difficult to polish perfectly, so the linear-prism extrusions typically have optical flaws inherent in manufacturing, which are similar to those of extruded optical fibers.
Extrusion flaws in prismatic extrusions are substantially removed in the manufacturing process for pyramidal-prismatic lens sheets for fluorescent lighting fixtures. Light control sheets of this type are typically made by a machine in which a flat extruded thermoplastic sheet, often up to four feet wide, is continuously extruded from an extrusion press. While the extruded sheet is still plastic, it passes between a pair of highly-polished embossing rollers having prismatic embossing patterns thereon. The rollers apply the embossed pyramidal prismatic surfaces to the soft, extruded sheet. The rollers emboss, polish, cool and rigidize the plastic sheet, forming a continuous prismatic sheet that is then sawn into rectangular panels for installation into fluorescent light fixtures. This is a high-speed process, in which the large-diameter rollers produce very accurate and highly-polished surfaces on the plastic. Prismatic lighting panel sheets of acrylic plastics are thus very inexpensively manufactured by this process, and are normally sold at a cost of less than $2 per pound.
The primary purpose of the present invention is to provide a manufacturing method for optical fibers that is a high-speed, high-volume process with the quality and cost advantages of pyramidal prismatic lighting sheet manufacturing, is combined with new rolling and cladding techniques to produce clad acrylic optical fibers at a significantly lower cost than presently-known fiber optics manufacturing methods.